This invention relates to mechanical wheel alarms for mounting to the hub of rolling equipment such as dump trucks, such alarms including one or more ringing units each specially constructed to ring only in a single rotary direction. More particularly, this invention relates to an improved ringing unit for a wheel hub mounted back up alarm having an improved clapper and an improved interacting keeper to sound a prompt audible alarm when the wheel rotates in one direction (normally the reverse direction), to prevent sounding of the alarm when the wheel rotates in the opposite direction (normally the forward direction), to regard alarm freezing, sticking, and other malfunction, and to provide minimum wear on non-sounding rotation.